Many applications exist for improved methods for removing polymers and other organic materials from substrates. For example, certain steps in the fabrication of semiconductor devices require etching of photoresists and cleaning of residual organic materials from surfaces. Many photoresists are based on polymers that have a large activation energy for etching reactions and thus are difficult to etch. Previously used dry etching methods for removing such materials have been characterized by various disadvantages and limitations including requirements for extreme and damaging reactive conditions, use of hazardous reactants and/or high temperatures, slow reactions and adverse loading effect.
One commonly used etching or thinning procedure for polymers or photoresists involves placing the sample to be etched directly in a plasma discharge. While such a procedure provides for effective etching, it presents a disadvantage in that the sample is exposed to highly energetic ions, electrons, photons and electric fields which can result in substrate damage, especially if the substrate is a semiconductor or integrated circuit. Electrostatic breakdown causing significant damage to semiconductor devices has been observed under the harsh conditions encountered in direct exposure to a plasma during photoresist stripping.
Another approach to etching or stripping of photoresists, which avoids the damaging direct exposure of the sample to the plasma, has been to establish a plasma discharge in one region of a reaction vessel, thus producing a reactive gas species in that region, with the sample being placed in a second region away from the plasma discharge. This type of process, known as downstream or afterglow etching, is exemplified by U.S. Pat. Nos. 4,368,092, Steinberg et al. and 4,554,047, Cook et al. Reaction rates obtained by this means are slow for difficult-to-etch polymers, and both of the cited patents require the use a hazardous reactant gas mixture, CF.sub.4 and O.sub.2, to enhance etch rates.
An etching process which makes use of a solid polymeric material as a source of reactive gas species is disclosed by Ahn et al. in U.S. Pat. No. 4,243,476. The solid material when struck by an ion beam releases reactive gases, which in turn react with and etch a sample. Various hydrocarbons and halogenated hydrocarbon polymers are disclosed for use as a solid material source, with halogenated polymers that release a highly reactive halogen-containing species being emphasized. While this process may result in enhanced etching rates, the solid material from which the reactive gas is obtained is located in the vicinity of the sample substrates so that both are subjected to bombardment by highly energetic ions. As pointed out above, direct exposure of a substrate to such conditions can result in substrate damage.
In addition to providing faster etching rates while avoiding conditions that may result in substrate damage, it is desired to avoid adverse results of the loading effect as described in the prior art. W. S. Ruska discloses in Microelectronic Processing: An Introduction to the Manufacture of Integrated Circuits, published by the MacGraw-Hill Company of New York, N.Y., in 1987, which is incorporated herein by reference, many aspects of processes to produce integrated circuits. Specifically, in Chapter 6, "Etching," Ruska discloses a number of specific reaction conditions to obtain etching. On page 221, he specifically discusses the loading effect of the etching process, as follows:
"One further complication of plasma etching is the loading effect. The rates of many reactions are found to depend on the area (A below) of substrate being etched: a greater exposed area to be etched slows the reaction. This results from depletion of the reactive species from the plasma by the etching reaction. Consider an etch rate, with rate constant k dependent on a reactive species with a lifetime in the plasma .tau. and a volume generation rate (e.g. in molecules per liter.sup.-1 sec.sup.-1) G. Subject to some very plausible assumptions, the etch rate can be written as ##EQU1## where r is the etch rate, A is the area of material being etched and K is a constant determined by the reaction and the reactor geometry. It follows that r depends on A if the product of reaction rate, K, and lifetime .tau. becomes sufficiently large. A loading effect can be avoided by choosing a chemistry with a slow reaction rate or a short species lifetime. This, however, results in a decrease in the maximum etch rate, given by K.tau. G unless the species generation rate G can be made correspondingly large. The result is an unavoidable tradeoff between reaction rate and loading effect. PA1 "Loading effect is intrinsically undesirable and becomes especially troublesome when good end-point control is crucial. As the etching process becomes almost complete, the area being etched decreases rapidly toward zero. If there is a substantial loading effect, the etch rate rises rapidly, resulting in a runaway etching of the remaining volume. If the reaction possesses any isotropy, the result can be excessive undercutting caused by fast etching of the small exposed area at the sides of the cuts."
The loading effect is also disclosed in the above-cited Cook et al. patent which is concerned with avoiding the effect by control of certain process and reactor features differing in many respects from the present invention. The loading effect mentioned in the prior art has been characterized by a decrease in sample etch rate with increased reactor loading.
Prior art processes are further exemplified by U.S. Pat. Nos. 4,412,119, Komatsu et al.; 3,692,655, Vossen Jr.; and 4,612,099, Tanno et al. These patents are concerned, respectively, with plasma etching of a sample within the plasma, radio frequency sputter etching, and reactive ion etching, all of them employing features that differ widely from the present invention.